In response to rapid development of electronic devices, peripheral parts used in electronic devices must have advanced properties. For example, with respect to a light-transmitting lens and light-transmitting window for an electronic device, what is required is not only high in-line transmittance but also a material having superior polarization property in order to transmit high density optical signals. Also, it is important to use a material having high heat-sink capacity since the functional stability of an electronic device is affected by subtle generation of heat.
In the past, sapphire has been well known as an optical material having excellent heat sink capacity. Sapphire consists of alumina single crystal, and has high strength as well as very high heat conductivity of 42 W/(m·K). However, sapphire is expensive and has hexagonal crystal structure, exhibiting optical anisotropy. Therefore, if sapphire is used for a transmitting window of a polarized light optical instrument, it takes much time and labor for packaging, resulting in high manufacturing cost, since the axial adjustment of the crystallographic axis of sapphire is necessary with respect to the direction of incident polarized light.
Quartz glass is known as a material having superior polarization property and in-line transmittance. However, the thermal conductivity of quartz glass is as small as 0.01 W/(m·K) to 0.9 W/(m·K) and the heat sink capacity of quartz glass is not sufficient. A material having excellent polarization property is a spinel sintered body. Since the spinel sintered body consists of Al2O3 and MgO and its crystal type is cubic crystal, it ideally does not exhibit birefringence and is superior in terms of polarization properties.
Of the spinel sintered body, it is introduced that the materials in which the composition ratio of Al2O3 and MgO is 0.53:0.47 to 0.58:0.42 have superior in-line transmittance (Refer to Japanese Patent Application Laid-Open No. S 59-121158 (Patent document 1)). It is disclosed that this composition ratio can be obtained when a material powder in which n is in the range of 1.127 to 1.381 in the expression of Al2O3.nMgO is subjected to pressureless sintering, after forming, at 1700° C. to 1800° C. for 10 hours to 20 hours in a hydrogen atmosphere.
According to the teaching disclosed in the above-mentioned patent document, in order to obtain a spinel sintered body having a high transmissivity by sintering in the hydrogen atmosphere, it is better to allow 10% or more of the MgO contained in the material powder to evaporate rather than completely restraining the vaporization of MgO, which vaporizes at a temperature of 1400° C. or higher. In the above-mentioned patent document, it is also suggested that preferably the sintering should be performed in a hydrogen atmosphere because transmitting light is scattered and accordingly the transmissivity deteriorates in any cases where the sintering is performed in air or N2 gas or in vacuum: if the sintering is performed in air or N2 gas, N2 will be introduced into a closed pore, which results in difficulty in the vanishment of the pore, whereas if the sintering is performed in vacuum, the vanishment of MgO becomes remarkable such that Al2O3 becomes overabundant.    [Patent document 1] Japanese Patent Application Laid-Open No. S59-121158